1. Field of the Invention
The present invention relates to a spatial light modulator used for an optical processor, a projection display, a rewritable hologram display, and the like, and a method for fabricating the same.
2. Description of the Related Art
A spatial light modulator (hereinafter, referred to as an SLM) of an optically addressable type having a modulating portion and a photoconductor layer (the SLM as used herein refers to this optically addressable type) have many functions including optical thresholding operation, wavelength conversion, incoherent/coherent conversion, and image storing. The SLM is therefore recognized as a key device for optical information processing. The SLM also has a function of optical amplification, allowing for the use for a projection display. It therefore has excellent versatility.
The modulating portion and the photoconductor layer of the SLM are sandwiched by two transparent conductive electrodes. The optical transmittance of the modulating portion changes depending on the voltage applied between the transparent conductive electrodes. The principle of the operation of the SLM is as follows: the photoconductor layer is illuminated by a writing light while a voltage is applied between the transparent conductive electrodes, so as to change the electric resistivity of the photoconductor layer. This causes a change in the voltage applied to the modulating portion, and by this change the transmittance of the modulating portion is modulated.
The most convenient type of such an SLM with high photosensitivity, high-speed response, and low operating voltage is the one using hydrogenated amorphous silicon (hereinafter, referred to as "a--Si:H") as the photoconductor layer and ferroelectric liquid crystal (hereinafter, referred to as FLC) as the modulating portion. Currently, this type of SLM has been most actively studied and developed. FIG. 18 shows a typical example of this type of the SLM (refer to G. Moddel et al., Appl. Phys. Lett., 55 (1989), p. 537). Referring to FIG. 18, the SLM includes a photoconductor layer 701 which is a pin diode made of a--Si:H and an FLC layer 702. The photoconductor layer 701 and the FLC layer 702 are sandwiched by transparent insulating substrates 705 and 706 on which transparent conductive electrodes 703 and 704 are formed, respectively.
As mentioned above, the SLM is applicable to a projection display by utilizing the function of optical amplification. In this application, an image is written into a photoconductor layer with a weak writing light, which is then transferred onto a liquid crystal layer (hereinafter, referred to an LC layer) and read therefrom with an intensive reading light, so as to project a magnified image on a large screen. In order to prevent the intensive reading light from leaking into the photoconductor layer, a reflecting mirror composed of multilayers of dielectrics is formed between the photoconductor layer and the LC layer. In order to enhance the resolution of the image to be read, U.S. Pat. No. 4,913,531, for example, describes the use of separated pixels of reflector 801 as the reflecting mirror as shown in FIG. 19.
However, the conventional SLM has disadvantages as described below. The photoconductor layer of the conventional SLM, which is formed of a diode, is illuminated by the writing light while a reverse bias is applied. This results in that only the current which flows in the photoconductor layer is the primary photocurrent. In other words, the amount of a photocurrent flowing in the photoconductor layer is determined by the number of photons entering the photoconductor layer when the photoconductor layer is illuminated by the writing light. This indicates that the current with a quantum efficiency of more than 1 does not flow in the photoconductor layer.
Further, the amount of charges for a unit area at the interface of the photoconductor layer and the LC layer required to induce a voltage large enough to switch the state of the LC layer depends on the liquid crystal material and the cell thickness. Accordingly, given the ideal conditions where the photocurrent with a quantum efficiency of 1 flows in the photoconductor layer at a response speed sufficiently higher than that of the liquid crystal, the intensity of the incident light (photosensitivity) required to switch the LC layer is independent of the properties of the photoconductor layer. The a--Si:H pin diode structure is known to provide these ideal conditions (refer to K. Akiyama et al., Jpn. J. Appl. Phys., 30 (1991) p. 3887, for example).
Since the photoconductor layer of the conventional SLM is already in such ideal conditions, it is impossible to seek for an SLM with higher photosensitivity unless a novel liquid crystal material having a higher response speed at a low-voltage operation can be developed. In the meantime, realization of an SLM with higher photosensitivity is a most critical issue in the field of optical computation which conducts a massively parallel processing. This is because, as the level of the parallel processing enhances, the total amount of writing light increases and thus a light source capable of supplying a larger power of light is required. This results in the light source to be used is determined on the required amount of light, and the power consumption of the light source, so, as a result the operation cost increases. For these reasons, higher photosensitivity in the SLM is strongly desired.
As mentioned earlier, the reflecting mirror composed of multilayers of dielectrics is formed between the photoconductor layer and the LC layer in order to prevent the reading light from leaking into the photoconductor layer. As shown in FIG. 19, to ensure the prevention of the leaking, a groove is formed in a portion of a photoconductor layer 802 corresponding to the gap between reflector pixels 801, and another reflector 803 made of Al or the like is formed in the groove. In this structure, however, short-circuiting tends to occur between the reflector pixels 801 and the reflector 803. On the other hand, without the reflector 803, the trouble of short-circuiting is prevented, but a number of photocarriers are generated around the groove in the photoconductor layer 802 by the illumination of an intensive reading light, and diffuse laterally in the photoconductor layer 802. This lowers the resistivity of portions of the photoconductor layer 802 covered with the reflector pixels 801, causing the LC layer to be switched on even when a writing light is not incident.